1. Field of the Invention
The present invention relates to the field of ion exchange composite membranes. The present invention particularly describes organic/inorganic hybrid composite proton exchange membranes which can be operated in a high temperature, low humidity environment.
2. Description of Related Art
Fuel cells (FC) are electrochemical devices that convert chemical energy directly into electrical energy. FC have been used as a power source in many applications and can provide improved efficiency, reliability, durability, cost and environmental benefits over other source of electrical energy. As a result of the improved operation of these FC over other sources of energy, and in particular, the reduced emissions (i.e., practically zero harmful emissions). The FC are applicable to various fields, such as a portable electronic product, a home power generation system, transportation, military equipment, space industry, a small-size power generation system, and so forth.
Specifically, various FCs can be applied to different fields based on different operational conditions. When the FC is used as a mobile energy source, the FC mainly refers to a hydrogen proton exchange membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC). Both of them are operated at low temperature with use of the proton exchange membrane to perform proton conduction mechanism.
Generally, the fuel cell comprises an anode, at which electron are catalytically removed from the fuel and fed to an external circuit, and proton are catalytically removed from the fuel and fed across a proton exchange membrane to a cathode, where the electrons, protons, and an oxidant are recombined to close the circuit. Taken PEMFC for example, hydrogen oxidation reaction takes place in an anode catalyst layer, such that hydrogen ions (H+) and electrons (e−) are generated. The hydrogen ions can be conducted to the cathode through a proton-conducting membrane, while the electrons can be transported to the cathode after the electrons flowing through an external circuit are applied to a load to work. Here, reduction reaction between the oxygen supplied to the cathode and the hydrogen ions and the electrons occurs at a cathode catalyst layer, and thereby water is produced. So the performance of the PEMFC relies on three-phase catalytic reaction efficiency, i.e. electron-conductivity, ion-conductivity, and fuel-activity that all matter to design of the FC. As long as any path of the three is hindered, the performance of the PEMFC is affected accordingly. Here, the ion-conductivity is mainly determined based on the proton exchange membrane.
Prior fuel cell systems typically utilize an externally humidified air stream to maintain the proper moisture level of the membranes of the MEA. However, providing water to the stack is costly from a system point of view, and it is desirable to supply as little water as possible in order to minimize system complexity and cost. Based on the above, the PEMFC tends to be equipped with a low humidification operating system. By contrast, passive operation of the DMFC is gradually developed. The key technology of the PEMFC and the DMFC lies in high water-retention capacity and the proton exchange membrane capable of conducting protons rapidly.
In the low humidification PEMFC, adsorbent materials embedded in the membrane which adsorb water under wet conditions and provide a reservoir of water to keep the membrane irrigated under dry conditions. Thus, the water adsorbing materials allow the fuel cell to survive periods of “inlet-stream draught” without excessive loss in conductivity. On the contrary, methanol with high concentration acts as fuel in the passive DMFC that is characterized by reduced system components and easy-to-carry methanol with high energy density. Besides, the methanol stays in the liquid state on all conditions and does not require a complex vaporization process and relative components for generating hydrogen gas in a direct FC.
Nevertheless, on the one hand the passive DMFC uses highly concentrated methanol vapor as fuel and on the other DMFC requires water as another anode fuel, so that insufficient water brings about an increase in ion conduction resistance. Thereby, performance of the passive DMFC is significantly deteriorated. On the other hand, since fuel with extremely low relative humidity is supplied to the PEMFC with the low humidification, there are issues of proton conduction resistance and peeling of the catalyst layer caused by ion exchange membrane shrinkage on a wet-dry operational condition. Hence, how to maintain the water-retention capacity, dimensional stability, electrochemical stability, chemical resistance, flexibility, and mechanical strength during the high temperature operation is the issue to be resolved no matter the passive DMFC using the highly concentrated methanol vapor or the hydrogen PEMFC with the low humidification operating system is employed. As such, it is a pressing need to develop a proton exchange membrane which is capable of conducting protons rapidly and characterized by high water-retention capacity and favorable dimensional stability.